The Class eTicket represents a single ticket data that is used both in HSL single tickets and in HSL travel card's value tickets.
The Class SingleTicket stores all the data read from the HSL single ticket.
The Class TravelCard stores all the data read from the HSL travel card.
Using these classes is simple, you just pass the raw data from the card to the constructor of TravelCard or SingleTicket which will extract the raw data into member variables that can be read through getter methods.
The meaning of the member variables content can be interpreted using separate documentation about HSL card's contents.